Superficial segregation, wetting, and dynamical equilibrium in bimetallic clusters: A Monte Carlo study

Abstract

Using Monte Carlo simulations on a lattice-gas model within the pseudo-grand-canonical ensemble, we study the competition between superficial segregation, wetting and a core dynamical equilibrium for nanoparticles made of thousands of atoms in a system that tends to phase separate, e.g., Cu-Ag. Increasing the chemical potential difference $\ensuremath{\Delta}\ensuremath{\mu}$ between Ag and Cu (or the nominal Ag concentration) at a temperature lower than the critical temperature for the phase separation in the infinite crystal, we show that the cluster goes through different stages: (i) Ag-superficial segregation that involves the vertices first, then the edges, and finally the (111) and (001) facets; (ii) prewetting that leads to Ag enrichment on the shells close to the cluster surface; (iii) a dynamical equilibrium that affects all the internal shells jointly, similar to the first-order phase transition due to the miscibility gap in an infinite crystal; and (iv) again standard segregation. Moreover, we show that a similar behavior occurs for the cluster facets if the temperature is lower than the critical temperatures of the first-order phase transition of the corresponding surfaces of semi-infinite crystals. A remarkable consequence of those dynamical equilibria is that very different concentrations of the facets on one hand and of the whole cluster on the other hand can be observed at a given $\ensuremath{\Delta}\ensuremath{\mu}$.